Global position systems, such as the American GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Defense Department is a satellite-based radio navigation system which transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with equal precision. The GPS uses 24 satellites dispersed in six, inclined, 12 hour circular orbits chosen to insure continuous 24 hour coverage world-wide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHZ and an L2 signal at a frequency of 1227.6 MHZ. The L1 and L2 signals are bi-phase modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigation data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN signal to recover the navigation data, including ephemeris data. The ephemeris data is used with a set of Kepler to precisely determine the location of each satellite.
The receiver measures a phase difference of time of arrival of signals from at least four satellites. The phase differences are used to solve a matrix of four equations. The result is a precise determination of location of the receiver in three-dimensional space.
Velocity of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measured differences are used to solve another set of equations to determine velocity based upon the Doppler phase shift of the received signal.
While the GPS performs relatively well, there are still situations where receivers cannot adequately detect satellite signals. For instance, where a receiver is located in deep valleys, between buildings in a city or in the presence of interference, a receiver cannot detect a sufficient number of satellite signals to accurately determine position. Further, during severe weather (e.g., during electrical or heavy rain storms), GPS signals may be so severely attenuated as to be undetectable. Accordingly, a need exists for a means for integrating GPS positioning with other, more conventional, positioning techniques and for means for selecting a reliable position solution from among those techniques.